Compositions containing polycarbonate (PC) and ABS (acrylonitrile/butadiene/styrene) and including alkylammonium montmorillonite preferentially distributed in the ABS phase were reported to have increased thermal stability by Wang et al. (Wang, S., Hu, Y., Wang, Z., Yong, T., Chen, Z., & Fan, W., Synthesis and Characterization of polycarbonate/ABS/montmorillonite nanocomposites, Polymer Degradation and Stability, 80, No. 1, (2003) 157-61).
It is also known that conventional alkylammonium modifiers, such as tallow fat-bis(2-hydroxyethyl)methylammonium and impurities contained therein (for example iron ions) decompose the polycarbonate matrix and, for example, increase the heat release rate (determined by means of cone calorimetry) (see Stretz, H. A., Koo, J. H., Dimas, V. M., & Zhang, Y., Flame retardant properties of polycarbonate/montmorillonite clay nanocomposite blends, Polymer Preprints, 42, no. 2, (2001) 50; Yoon, P. J., Hunter, D. L. & Paul, D. R., Polycarbonate nanocomposites: Part 2. Degradation and color formation, Polymer, 44, no. 18, (2003) 5341-54).
The synergistic effect of alkylammonium montmorillonites for flame retardant properties is known for ABS molding compositions (Wang, S., Hu, Y., Zong, R., Tang, Y., Chen, Z., & Fan, W., Preparation and characterization of flame retardant ABS/montmorillonite nanocomposite. Applied Clay Science, 25, no. 1-2, (2004) 49-55). Wang et al. achieve improved flame retardant properties for ABS moldings in combination with antimony oxide and decabromodiphenyl oxide. In the presence of the alkylammonium montmorillonite, the heat release rate (cone calorimeter) was reduced and the time to ignition was prolonged, i.e. the LOI (limiting oxygen index) was higher and a flammability rating of V-0 in the UL94 V test was achieved.
WO 99/43747 A1 discloses the synergistic effect for flame retardant properties of alkylammonium montmorillonites for PC/ABS compositions. The time to ignition of PC/ABS compositions is prolonged by the addition of alkylammonium montmorillonite.
A disadvantage of the alkylammonium-modified montmorillonites is the involved and expensive modification process for preparing the layered silicate. Layered silicates modified in this way also have an adverse effect on the physical properties of polycarbonate compositions, since the basic modifiers of the layered silicate degrade the polymer and thus lead to a reduction in the molecular weight of the polymer and to clouding and discoloration. U.S. 2005/0137287 A1 discloses polycarbonate compositions which comprise layered silicates modified with a block copolymer of 2-(dimethylamino)-styrene/ethyl methacrylate with a quaternary ammonium end group. The resulting polycarbonate molding compositions are transparent and show no discoloration.
WO 99/07790 A1 and Fischer et al. (Fischer, H. R., Gielgens, L. H., & Koster, T. P. M., Nanocomposites from polymers and layered minerals, Mat. Res. Soc. Proc. vol. 519, 1998, 117-123) disclose nanocomposite materials which comprise block or graft copolymers and a layered silicate, the copolymer carrying a structural unit which is compatible with the layered silicate and a further unit which is compatible with the polymer matrix. In the first step of the preparation of the composite materials the layered silicate is mixed with the copolymer at elevated temperature, and in a subsequent step by extrusion in the polymer matrix sought. Alternatively, a solvent can also be added. An improvement in the tensile strength of the modified material is achieved in this way. PC/ABS compositions and modification processes for the layered silicates via the aqueous route are not described.
Disadvantages of modification by means of block copolymers are the involved and expensive modification processes and the need for an additional process step for synthesizing the block copolymer.
The object on which the invention is based is to provide polycarbonate molding compositions having a high thermal stability, a low maximum rate of decomposition after ignition and a low smoke density.